1. Field of the Invention
The object of the invention is an exhaust gas aftertreatment system in internal combustion engines operated with a lean mixture, such as diesel engines and gasoline engines with direct injection, wherein the nitrogen oxides are reduced by means of an SCR catalyst, and the particulates are reduced by means of a particle separator or a particle filter.
2. Description of the Related Art
In addition to solid particulates, nitrogen oxides are among the limited exhaust gas components which are formed during combustion processes and whose allowed emissions are being reduced to ever lower levels. To minimize these exhaust gas components in internal combustion engines operated in motor vehicles, various methods are now being used. The lowering of nitrogen oxide levels is usually accomplished with catalysts; in oxygen-rich exhaust gas, a reducing agent is additionally needed to increase the selectivity and the NOx conversion rates. These processes are collectively known as SCR processes (SCR=selective catalytic reduction). They have been used for many years in power plants and more recently in internal combustion engines. DE 34 28 232 A1 provides a detailed description of processes of this type. Examples of suitable SCR catalysts include mixed-metal oxides that contain V2O5, for example, in the form of V2O5/WO3/TiO2. V2O5 is typically present in these catalysts in amounts of 0.2 to 3%. Reducing agents that have found practical use are ammonia or ammonia-cleaving compounds, such as urea or ammonium formate in solid or dissolved form. In this regard, 1 mole of ammonia is required for the reaction of 1 mole of nitric oxide.4NO+4NH3+O2→4N2+6H2O  (1)
If a platinum-containing NO oxidation catalyst for forming NO2 is used upstream of the SCR catalysts,2NO+O22NO2  (2)the SCR reaction can be considerably accelerated, and the low-temperature activity can be appreciably increased.NO+2NH3→2N2+3H2O  (3)
In internal combustion engines operated in motor vehicles, nitrogen oxide reduction by means of the SCR process turns out to be difficult, because varying operating conditions make the quantitative metering of the reducing agent difficult. On the one hand, it is desired that the greatest possible conversion of nitrogen oxides be realized, but on the other hand, it is necessary to make sure that there is no emission of unconsumed ammonia. To this end, an ammonia-blocking catalyst is often used downstream of the SCR catalyst to react with excess ammonia to form nitrogen and water vapor. In addition, the use of V2O5 as the active material for the SCR catalyst can then lead to deactivation problems if the exhaust gas temperature at the SCR catalyst is greater than 650° C. For this reason, V2O5-free iron or copper zeolites are used for high-temperature applications.
To minimize the solid particulates, either so-called particle separators or particle filters are used both in the power plant sector and in motor vehicles. A typical system with a particle separator for use in motor vehicles is described, for example, in EP 1 072 765 A1. Systems of this type differ from systems with particle filters in that the diameter of the channels of the particle separator is much greater than the diameter of the largest particles that are present, whereas in particle filters the diameter of the filter channels is of the same order of magnitude as the diameter of the particles. As a result of this difference, particle filters are prone to clogging, which increases exhaust gas back pressure and reduces engine output. Particle separators of the aforementioned type can be improved if, as described in US 2003/0072694 the exhaust gas can also flow through the individual filter layers transversely to the main direction of flow of the exhaust gas. In this way, thorough mixing and thus homogenization of the exhaust gas stream are realized. U.S. Pat. No. 4,902,487 describes a system and a process that use a particle filter instead of a particle separator of the type described above. The aforementioned systems and processes are distinguished by the fact that the oxidation catalyst, which is usually a catalyst that contains platinum as the active material and is located upstream of the particle separator or particle filter, oxidizes the nitric oxide in the exhaust gas with the aid of the residual oxygen that is also present to form nitrogen dioxide, which in turn reacts with the carbon particulates in the particle separator or particle filter to form CO, CO2, N2, and NO. In this way, there is continuous removal of the deposited solid particulates, and regeneration cycles, which must be carried out by complicated means in other systems, are thus eliminated.2NO2+C→2NO+CO2  (4)NO2+C→NO+CO  (5)2C+2NO2→N2+2CO2  (6)
To comply with future exhaust gas regulations, the simultaneous use of systems for reducing nitrogen oxide emissions and systems for reducing solid particulate emissions is necessary. Various systems and processes for this are already known.
U.S. Pat. No. 6,928,806 describes a system that consists of an oxidation catalyst, an SCR catalyst downstream of it, and a particle filter downstream of the SCR catalyst in the exhaust gas stream. The reducing agent for the selective catalytic reaction that takes place in the SCR catalyst is supplied directly upstream of the SCR catalyst by means of a urea injection device controlled as a function of operating parameters of the internal combustion engine. A disadvantage of this system is that the nitrogen dioxide produced in the oxidation catalyst is virtually completely consumed by the selective catalytic reduction in the SCR catalyst and thus is not available for the reaction of the solid particulates deposited in the downstream particle filter. Therefore, the regeneration of the particle filter must be accomplished in a complicated way by cyclical heating of the exhaust gas stream by enriching the exhaust gas stream with uncombusted hydrocarbons. This is accomplished either by adding oil to the combustion mixture or injecting fuel upstream of the particle filter. On the one hand, a system of this type for regenerating the particle filter is complicated and thus expensive, and, on the other hand, the cyclical regeneration of the particle filter, which is located at the end of the system, produces further foreign substances that can no longer be removed from the exhaust gas. In addition, when particle filters are used, the filters can become clogged by oil incineration ash, so that the filters must be removed and cleaned at regular intervals.
Another combination of a particle filter and a system for selective catalytic reduction is disclosed by U.S. Pat. No. 6,805,849. The system described there consists of an oxidation catalyst installed in the exhaust gas stream, which increases the fraction of nitrogen dioxide in the exhaust gas, a fine particle filter downstream of the oxidation catalyst, a reservoir for the reductant fluid, an injection device for the reductant fluid, which is located downstream of the fine particle filter, and an SCR catalyst located in the stream of exhaust gas downstream of the point of injection of the reductant fluid. Although the system described above allows continuous reaction of the solid particulates (soot) deposited in the fine particle filter by means of the nitrogen dioxide produced in the oxidation catalyst, it has a serious disadvantage. The particle filter causes cooling of the exhaust gases, so that, e.g., when the now standard commercial reductant fluid known as AdBlue is used, the exhaust gas temperature is too low, especially when the internal combustion engine is in the start-up phase or is being operated in the low power range, to produce ammonia from the 33% aqueous urea solution without the simultaneous production of problematic byproducts.
In connection with the decomposition of urea ((NH2)2CO) into ammonia (NH3), it is well known that this occurs in two stages under optimum conditions (temperatures above 350° C.). First, thermolysis, i.e., thermal decomposition, of urea occurs according to the following equation:(NH2)2CO→NH3+HNCO  (7)
Hydrolysis then occurs to a small extent, i.e., decomposition of isocyanic acid (HNCO) to ammonia (NH3) and carbon dioxide (CO2) according to the following equation:HNCO+H2O→NH3+CO2  (8)
Since the reducing agent is present in the form of an aqueous solution when AdBlue is used, this water must evaporate before and during the actual thermolysis and hydrolysis.
If the temperatures prevailing in the above reactions according to equations (7) and (8) are below 350° C. or if heating takes place at only a slow rate, it is known from DE 40 38 054 A1 that principally solid, infusible cyanuric acid is formed by trimerization of the isocyanic acid formed by (7) in accordance with the following equation:
which causes clogging of the downstream SCR catalyst. As noted in the above-cited DE 40 38 054 A1, this problem can be remedied by passing the stream of exhaust gas, which is loaded with the reducing agent, over a urea decomposition and hydrolysis catalyst. The exhaust gas temperature at which quantitative hydrolysis becomes possible can be depressed to 160° C. in this way. The design and composition of a corresponding catalyst are described in the cited publication, as are the design and function of an SCR catalyst system provided with a hydrolysis catalyst.